I have a 1995 k1500 that I have been having problems with the heat control. I started off with a clicking noise under the dash and heat would go off and on and same with speed of blower. I replace resistor and blower and that solved the blower speed problems, but not heat going from hot to cool and back to hot by itself. Found out noise to be broken fresh air actuator. I disconnected it and just left it be. I still have heat problem. In the mist of this I ended up with a cracked radiator and replaced that. At that time I replace engine temp sensor and thermostat. Still have heat problem. Took glovebox out and watched blend actuator. It turns properly as I turn the temp switch, but when on hot it keeps trying to go back to cool. Not sure if the actuator is bad, harness, or the whole control head. I can't afford to start replacing things without a better idea of what to try.
Any advice would be helpfull. Thanks

 

i am having a similar issue but mine only goes to the floor not the defrost or dash think it is the control unit have heard that these are prone to failure